Until very recently, asbestos has been the standard, most widely used material in braided "compression" packings. The recent discovery of the potential carcinogenic effects, as well as the fact that supplies of the raw material are naturally limited as well as not widely distributed geographically and, hence, subject to political restraints, has caused a wide search for alternative materials in the fluid sealing industry. In a patent issued to one of us (U.S. Pat. No. 3,306,155), it was disclosed that the substitute material, namely, glass fiber, in combination with polytetrafluoroethylene (TFE) dispersion, forms a braided packing which is effective for a number of applications. However, in order to prepare a TFE dispersion, it is necessary to incorporate a wetting agent which then becomes residual in the braided structure. Where the glass fiber packing is used in contact with water or with an aqueous solution, the presence of the wetting agent makes it possible for the solution to gradually wash out the TFE lubricant after which self-abrasion of the glass fiber takes place rapidly. Where the aqueous solution is under pressure, the elution of the lubricant takes place even more quickly.
While each of the substitute materials on the market, such as glass fiber in combination with TFE dispersion, TFE fiber, graphite filament, etc., has substantial functional merit, these combinations are considerably more expensive than the standard graphited-asbestos packing which has been the predominant braided packing for many years. The search for an economical substitute which will be equivalent in cost to graphited-asbestos packing, has not yet been successful.
Attempts have been made to incorporate graphite lubricant or other inorganic lubricants into a braided glass fiber structure. These attempts have hitherto yielded unsatisfactory results. Dry, flake graphite will not be retained by a braided glass fiber structure. When the graphite is mixed with an oil lubricant, the oil will seep out and will not be retained; furthermore, the use of oil limits the effective operating temperature range of the packing. Also, the wet, dripping packing so constructed would create both housekeeping and safety problems. Adding graphite to a more coherent, waxy lubricant would help to retain the graphite in the structure, but this mixture would fundamentally amount to a wax filler in which the graphite would have no significant value; furthermore the wax would limit the effective temperature of the packing to a very low range. An aqueous graphite dispersion also will not be retained in the braided glass fiber structure.
Experience derived from graphited-asbestos packing offers no clear path to the development of a viable graphited glass fiber packing. The ordinary asbestos yarn of commercial grade or better used in the manufacture of braided packing contains up to 25% or even more of a cotton or rayon binder which readily absorbs and forms a reservoir of lubricant, permitting the finished product to retain the lubricant within the braided structure indefinitely. Furthermore, the inherent structure of asbestos, containing multifarious voids as the result of uneven and very small and varied fibers which are incorporated into the yarn (as opposed to the smooth, regular filamentary nature of glass fiber), permits the retention of graphite-laden lubricants (or mica-laden, talc-laden or other particulate-laden mixtures) as well as flake graphite in dry, powdered form. Thus, braided asbestos packing has been easy and convenient to load with lubricants and the technique of doing so is well known.
Attempts to use the same technique on braided glass fiber have met with failure owing to the difference between the inherent characteristics of this fiber and of asbestos fiber. These differences can readily be seen from the following table E and table G taken from pages 10 and 11 of the "Handbook of Asbestos Textiles," third edition, published by the Asbestos Textile Institute. Table E shows that asbestos fiber has a surface area which is as much as 70 times as great as that of nylon. Moreover, since as shown in table G, the diameter of glass fiber is roughly the same as that of nylon, the surface area of asbestos is also up to 70 times as great as that of glass fiber. It can readily be seen why graphite adheres so much more strongly to asbestos fiber than it does to glass fiber.
Lately, attempts have been made to use more sophisticated, aqueous-based graphite-laden dispersions for the same purpose. Such dispersions are available from Joseph Dixon Co., and, differing from the traditional mixture of graphite and oil, they do appreciably penetrate the glass fiber structure. However, similar difficulties have been found with such materials when used with glass or other inorganic fibers. They tend to wear off or, under pressure, blow out of a glass-fiber braided structure more readily than from braided asbestos, since they are not held and protected by the same irregular fibrous structure nor are they suspended in a retained lubricant vehicle. Also, when such dispersions are applied to glass fiber and the packing dries, a hairy, brush-like surface emerges on the outer surface of the packing. Since a prime desideratum of any packing is a smooth, antifrictional surface, such a packing becomes highly suspect from a tactile point of view to the normal user who is accustomed to use "smoothness" of finish as one of the criteria for packing evaluation.
TABLE E ______________________________________ COMPARISON OF SURFACE AREA OF VARIOUS FIBERS WITH ASBESTOS* SURFACE AREA BY N.sub.2 ADSORPTION TYPE OF FIBER (SQ. CM./GRAM) ______________________________________ Nylon 3,100 Acetate Rayon 3,100 Cotton 7,200 Silk 7,600 Wool 9,600 Viscose Rayon 9,800 Asbestos 130,000 to 200,000+ (Chrysotile) ______________________________________ *Canadian Mining Metallurgical Bulletin, April, 1951 +Recent studies show that the maximum surface area may run as high as 500,000 sq. cm./gram.
TABLE G ______________________________________ COMPARISON OF DIAMETERS OF VARIOUS FIBERS WITH ASBESTOS* TYPE OF FIBER DIAMETER FIBRILS IN ONE FIBER (INCHES) LINEAR INCH ______________________________________ Human Hair 0.00158 630 Ramie 0.000985 1,015 Wool 0.0008 to 0.0011 910 to 1,250 Cotton 0.0004 2,500 Rayon 0.0003 3,300 Nylon 0.0003 3,300 Glass 0.00026 3,840 Rock Wool 0.000142 to 0.000284 3,520 to 7,040 Asbestos 0.000000706 to 0.00000118 850,000 to 1,400,000 (Chrysotile) ______________________________________ *Canadian Mining and Metallurgical Bulletin, April, 1951.
Accordingly, it would be highly desirable to develop a non-asbestos packing which enjoys the advantage of resilience contributed by suitable filaments and in which the solid lubricant is relatively inexpensive, and, most important, in which the solid lubricant is retained, even when subjected to contact with water or steam. The packing taught herein meets these objectives.